1. Field of the Invention
The present invention relates to the construction of a secondary battery module, of which a lithium ion battery is representative.
2. Description of Related Art
In the prior art, in many cases, with the object of increasing the load voltage or the load capacity, a module construction for an assembled battery is adopted in which a plurality of battery cells are connected in series, in parallel, or in various combinations of series and parallel, and these battery cells are housed within a casing. Moreover, from the point of view of safety, it is desirable to adopt a sealed construction in which harmful liquid, gas, smoke, flame and so on are prevented from escaping to the exterior of the battery.
There is a per se known module construction for a battery in which a plurality of battery cells are closely disposed and tightened together and are housed in a hard case. However, with only this construction, the heat that is evolved by the cells of the battery themselves is not radiated away well but builds up in the battery, so that the temperature of the battery rises to a high level, and this undesirably shortens its working life.
In recent years, there has been an increase in battery applications that employ accelerated charging and high rate discharge, so that it is easy for the battery temperature to become high, and this is yet a further cause for the working life to become shortened. In particular, if an assembled battery is constructed with flattened type battery cells so that the largest area sides of these cells are in mutual contact, then there is almost no path for heat dissipation other than the tab leads, so that the temperatures of the cells that are positioned at the central portion of the battery become extremely high, and they arrive at the ends of their working lives earlier than the cells at other positions, and as a result the overall life of the battery module becomes undesirably short. Due to this, various methods have recently been contemplated for cooling a battery module.
One background technique in this field is Japanese Laid-Open Patent Publication 2000-251951. Referring to the Abstract of this publication, we find the following description: “In a sealed group type secondary battery in which a plurality of battery cells are arranged in series, along with providing cooling medium passage portions on both sides of the battery cells along their direction of arrangement, also a cooling medium passage is formed between the cell casings and communicates between the battery cell casings with the cooling medium passage portions on both sides, so that all of the sides of the battery cells and also the portions between the battery cells are forcibly cooled with the cooling medium”.
Another publication in this field is Japanese Laid-Open Patent Publication 2009-140714. Referring to the Abstract of this publication, we find the following description: “An assembled battery module is characterized in which a plurality of flattened type heat pipes are arranged in parallel between one flattened type battery cell and another flattened type battery cell, with each of these flattened type heat pipes being contacted to an externally fitted case that serves as a heat sink”.